An electric power steering apparatus (EPS) which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the steering assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through reduction gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a current command value and a detected motor current value becomes small, and the adjustment of the voltage supplied to the motor is generally performed by an adjustment of a duty ratio of pulse width modulation (PWM) control.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel 1 is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, tie rods 6a and 6b, further via hub units 7a and 7b. In addition, a torsion bar (not shown) is inserted into the column shaft 2, the column shaft 2 is provided with a steering angle sensor 14 for detecting a steering angel θ of the steering wheel 1 in accordance with a twist angle of the torsion bar and a torque sensor 10 for detecting a steering torque Th, and a motor 20 for assisting a steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist (steering assist) command based on the steering torque Th detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 by means of a voltage control value Vref obtained by performing compensation or the like to the current command value. Moreover, it is possible to receive the vehicle speed Vel from a controller area network (CAN) or the like.
Further, the steering angle sensor 14 is not essential, it does not need to be provided, and it is possible to obtain the steering angle from a rotation sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to exchanging various information of a vehicle is connected to the control unit 30, and it is also possible to receive the vehicle speed Vel from the CAN 40. Further, it is also possible to connect a non-CAN 41 exchanging a communication, analog/digital signals, a radio wave or the like except the CAN 40 to the control unit 30.
The control unit 30 mainly comprises a CPU (including an MCU, an MPU and so on), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Th detected by the torque sensor 10 and the vehicle speed Vel detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1 for which the vehicle speed Vel is a parameter by using an assist map. The calculated current command value Iref1 is phase-compensated in a phase compensating section 32 for enhancing stability of a steering system, and a phase-compensated current command value Iref2 is inputted into an adding section 33. The steering torque Th is inputted into a differential compensating section 35 being a feedforward system for enhancing a response speed, and a differential-compensated steering torque Thd is inputted into the adding section 33. The adding section 33 adds the current command value Iref2 and the steering torque Thd, and a current command value Iref3 being the addition result is inputted into a subtracting section 34.
The subtracting section 34 obtains a deviation Iref4 (=Iref3−Im) between the current command value Iref3 and a motor current value Im being fed back. Proportional integral (PI) control or the like is applied to the deviation Iref4 in a current control section 36, the voltage control value Vref is inputted into a PWM-control section 37, where the duty ratio is calculated, and the motor 20 is PWM-driven through an inverter 38. The motor current value Im of the motor 20 is detected by a motor current detector 39 and is fed back to the subtracting section 34.
In such an electric power steering apparatus, a method of performing feedback by using a self-aligning torque (SAT) has been conventionally proposed as a measure for improving on-center feeling, for example, as disclosed in WO 2011/101979 (Patent Document 1).